1. Field of the Invention
The present invention relates to a thrust dynamic pressure bearing apparatus that rotatably supports a shaft member and a bearing member in a thrust direction by a dynamic pressure of a lubrication fluid, and a method for manufacturing a thrust dynamic pressure bearing apparatus.
2. Description of Related Art
In recent years, various proposals have been made for dynamic pressure bearing apparatuses that rotatably support rotor members such as magnetic discs, polygon mirrors, optical discs and the like at high speed. For example, FIG. 13 shows a thrust bearing section of a typical dynamic pressure bearing. The thrust bearing section includes a shaft member 11 that is connected to a thrust plate 2 and a thrust bearing member that is formed from a bearing sleeve 3 and a counter plate 4. The thrust plate 2 has dynamic pressure surfaces disposed opposite, in an axial direction of the shaft member 11, to a dynamic pressure surface of the bearing sleeve 3 and a dynamic pressure surface of the counter plate 4 with a specified gap provided between these dynamic pressure surfaces. Lubrication fluid such as air, oil or the like is filled in the gaps. Dynamic pressure generation grooves (not shown) are formed in at least one set of the dynamic pressure surfaces. As the shaft member rotates, the dynamic pressure generation grooves causes a dynamic pressure action to pressurize the lubrication fluid. As a result, dynamic pressure is generated in the lubrication fluid such that the shaft member 1 is floated and rotatably supported by the dynamic pressure with respect to the counter plate 4.
The thrust plate 2 used in this type of thrust dynamic pressure bearing apparatus has an affixing aperture 2a formed in its central area. The shaft member 1 is attached to the thrust plate 2 by an insertion connection means that may be provided by pressure inserting or shrink-fitting the shaft member 1 in the aperture 2a of the thrust plate 2. The thrust plate 2 contacts the shaft member 1 along a fixing connection section having an axial length Lh. The insertion connection means causes a compression stress working in a direction from an internal wall surface of the aperture 2a of the thrust plate 2 toward an external peripheral surface of the shaft member 1, such that the stress of the thrust plate 2 works in a radial direction toward the center of the shaft member 1. As a result, the thrust plate 2 strongly compresses the shaft member 1 along the fixing connection section having the axial length Lh.
Generally, a shaft end section of the shaft member 1 or open sections of the aperture 2a on both end faces of the thrust plate 2 are formed with chamfered sections 5, 6 and 7, respectively. Each of the chamfered sections 5, 6 and 7 is linear or curved in cross section. The chamfered sections 5, 6 and 7 are formed by diagonally cutting an edge of the shaft end section of the shaft member 1, and peripheral edges of the open sections of the aperture 2a of the thrust plate 2. By the provision of the chamfered sections 5, 6 and 7, an end area of the shaft end section of the shaft member 1 reduces in the radial direction toward the tip of the shaft member 1, and an opening area of the aperture 2a outwardly expands. As a result, the chamfered sections 5, 6 and 7 facilitate insertion of the shaft member 1 in the thrust plate 2.
As describe above, the chamfered sections 5, 6 and 7 are provided in the shaft member 1 and the thrust plate 2 to facilitate insertion of the shaft member 1 in the thrust plate 2. However, when the shaft member 1 and the thrust plate 2 having the chamfer structure described above are used to form a thrust dynamic pressure bearing section, satisfactory dynamic pressure characteristics are not obtained. The inventors of the present invention have discovered that the unsatisfactory dynamic pressure characteristics are caused by the following factors:
When the chamfered sections 5, 6 and 7 in the shaft member 1 and the thrust plate 2 have sizes that are outside a specified range of relative dimensional differences, the action of the compression stress in the thrust plate 2 that is created by the pressure fitting or the shrink-fitting becomes asymmetrical in the axial direction. As a result, the thrust plate 2 deforms itself. By the deformation, the thrust plate 2 warps in an arc shape or an opened umbrella shape toward one of the chamfered sections 5, 6, and 7 that has the largest size.
This phenomenon occurs even when the shaft member 1 does not have a chamfered section 5, and only the thrust plate 2 is provided with chamfer sections 6 and 7.
When the thrust plate 2 is deformed for the reasons described above, the perpendicularity between the shaft member 1 and the thrust plate 2 may be lost, and the thrust plate 2 cannot remain in parallel with the thrust bearing members 3 and 4, with the result that the dynamic pressure characteristic is substantially deteriorated. Also, the gaps in the thrust dynamic pressure bearing section become uneven in the peripheral direction. As a result, the balance of the pumping force generated by the fluid pressurizing means such as the dynamic pressure generation grooves is lost, and a specified float amount cannot be obtained. Furthermore, the minimum rotation speed that is required to float the shaft member 1 and the thrust plate 2 increases. In the worst case, the shaft member 1 and the thrust plate 2 may contact with the sleeve member 3 and/or the counter plate 4, with the result that the wear of the moving parts is promoted and the service life of the dynamic pressure bearing apparatus is shortened.
It is an object of the present invention to provide a thrust dynamic pressure bearing apparatus and a method of manufacturing a thrust dynamic pressure bearing apparatus that optimizes the dynamic pressure characteristics of a thrust dynamic pressure bearing section of the thrust dynamic pressure bearing apparatus.
In accordance with one embodiment of the present invention, a thrust dynamic pressure bearing apparatus includes a shaft member and a thrust plate having a fixing hole that is provided with chamfered sections Ca and Cb at the fixing hole. The shaft member is shrink fitted or pressure inserted in the fixing hole of the thrust plate. The chamfered sections Ca and Cb have axial lengths La and Lb in the axial direction. Dimensional relations of the axial lengths La and Lb in the chamfered sections Ca and Cb are set to be within a specified range.
In accordance with another embodiment of the present invention, a thrust dynamic pressure bearing apparatus includes a shaft member and a thrust plate having a fixing hole that is provided with chamfered sections at the fixing hole. The shaft member is shrink fitted or pressure inserted in the fixing hole of the thrust plate. The internal surface of the fixing hole engages the external surface of the shaft member along a fixing connection section having a length Lh. Axial distances La and Lb measured from end faces of the thrust plate to ends of the fixing connection section in the axial direction are set within a specified range with respect to the length Lh. As a result, the bonding force of the thrust plate applied to the shaft member is evenly distributed along the axial direction, and deformation of the thrust plate is suppressed to a very small amount in a range that does not affect the dynamic pressure characteristics. As a result, the deformation of the thrust plate can be well prevented by a relatively simple structure, and good thrust dynamic pressure characteristics can be maintained for a long time. More specifically, nonsynchronous oscillation (NRRO) is reduced and good bearing characteristics can be obtained, abrasion of parts within the bearing assembly is prevented, and the service life of the bearing is extended. Accordingly, the reliability of the thrust dynamic pressure bearing apparatus can be substantially improved.
Other features and advantages of the invention will be apparent from the following detailed description, taken in conjunction with the accompanying drawings that illustrate, by way of example, various features of embodiments of the invention.